Add mouseovers to D3 graphs on homepage

[aaronhans]

The D3 powered map and graphs on the first page of water.openoakland.org could be more usable if the bars on the charts and regions on the maps show tooltips when you mouseover them

This issue was suggested by Alissa Rubin:
"graphs and map should have mouse-over information showing what system(s) are indicated by each bar, what number of violation counts they are at, and how many systems are blocked together at that number of violations"